In general, an air conditioner for a vehicle includes a cooling system for cooling the interior of the vehicle and a heating system for heating the interior of the vehicle.
At an evaporator side of a refrigerant cycle, the cooling system converts air into cold air by exchanging heat between the air passing outside an evaporator and refrigerant flowing inside the evaporator so as to cool the interior of the vehicle. At a heater core side of a coolant cycle, the heating system convers air into warm air by exchanging heat between the air passing outside the heater core and coolant flowing inside the heater core so as to heat the interior of the vehicle.
In the meantime, differently from the air conditioner for the vehicle, a heat pump system which can selectively carry out cooling and heating by converting a flow direction of refrigerant using one refrigerant cycle has been applied. For instance, the heat pump system includes two heat exchangers: one being an internal heat exchanger mounted inside an air-conditioning case for exchanging heat with air blown to the interior of the vehicle; and the other one being an external heat exchanger for exchanging heat outside the air-conditioning case, and a direction changing valve for changing a flow direction of refrigerant.
Therefore, according to the flow direction of the refrigerant by the direction changing valve, the internal heat exchanger serves as a heat exchanger for cooling when the cooling mode is operated, and serves as a heat exchanger for heating when the heating mode is operated.
Various kinds of the heat pump system for the vehicle have been proposed, and FIG. 1 illustrates a representative example of the heat pump system for the vehicle.
As shown in FIG. 1, the heat pump system for the vehicle includes: a compressor 30 for compressing and discharging refrigerant; an internal heat exchanger 32 for radiating heat of the refrigerant discharged from the compressor 30; a first expansion valve 34 and a first bypass valve 36 mounted in parallel for selectively passing the refrigerant passing through the internal heat exchanger 32; an external heat exchanger 48 for exchanging heat with the refrigerant passing through the first expansion valve 34 or the first bypass valve 36 outdoors; an evaporator 60 for evaporating the refrigerant passing through the external heat exchanger 48; an accumulator 62 for dividing the refrigerant passing through the evaporator 60 into a gas-phase refrigerant and a liquid-phase refrigerant; a second expansion valve 56 for selectively expanding the refrigerant supplied to the evaporator 60; and a second bypass valve 58 mounted in parallel with the second expansion valve 56 for selectively connecting an outlet side of the external heat exchanger 48 and an inlet side of the accumulator 62.
Moreover, a chiller 50 which exchanges heat between refrigerant and coolant is mounted at the outlet side of the external heat exchanger 48.
In FIG. 1, the reference numeral 10 designates an air-conditioning case in which the internal heat exchanger 32 and the evaporator 60 are embedded, the reference numeral 12 designates a temperature-adjustable door for regulating a mixed amount of cold air and warm air, and the reference numeral 20 designates a blower mounted at an inlet of the air-conditioning case.
According to the heat pump system having the above structure, when a heat pump mode (heating mode) is operated, the first bypass valve 36 and the second expansion valve 56 are closed, and the first expansion valve 34 and the second bypass valve 58 are opened. Moreover, the temperature-adjustable door 12 is operated as shown in FIG. 1. Accordingly, the refrigerant discharged from the compressor 30 passes through the internal heat exchanger 32, the first expansion valve 34, the external heat exchanger 48, the chiller 50, the second bypass valve 58 and the accumulator 62 in order, and then, is returned to the compressor 30. That is, the internal heat exchanger 32 serves as a heater and the external heat exchanger 48 serves as an evaporator.
When an air-conditioning mode (cooling mode) is operated, the first bypass valve 36 and the second expansion valve 56 are opened, and the first expansion valve 34 and the second bypass valve 58 are closed. Furthermore, the temperature-adjustable door 12 closes a path of the internal heat exchanger 32. Therefore, the refrigerant discharged from the compressor 30 passes through the internal heat exchanger 32, the first bypass valve 36, the external heat exchanger 48, the chiller 50, the second expansion valve 56, the evaporator 60 and the accumulator 62 in order, and then, is returned to the compressor 30. That is, the evaporator 360 serves as an evaporator and the internal heat exchanger 32 closed by the temperature-adjustable door 12 serves as a heater in the same with the heat pump mode.
However, in case of the conventional heat pump system for the vehicle, in the heat pump mode (heating mode), the internal heat exchanger 32 mounted inside the air-conditioning case 10 serves as a heater to carry out heating and the external heat exchanger 48 mounted outside the air-conditioning case 10, namely, in front of the engine room of the vehicle, serves as an evaporator to exchange heat with outdoor air.
In this instance, if temperature of refrigerant flown into the external heat exchanger 48 is higher than the outdoor air, in other words, if outdoor air temperature is lower than the refrigerant temperature, the conventional heat pump system for the vehicle has a disadvantage in that the external heat exchanger 48 is deteriorated in heat exchange efficiency, for instance, the external heat exchanger cannot absorb heat from the outdoor air and frosting is formed on the external heat exchanger 48. So, the conventional heat pump system is deteriorated in heating performance and efficiency and it is impossible to operate the heat pump mode if outdoor air temperature is zero or lower.
Furthermore, in order to enhance performance of the air-cooling type external heat exchanger, thickness or an effective area for heat exchange must be increased to increase size of the external heat exchanger. However, the conventional external heat exchanger has a limit in increasing size and enhancing performance due to a small space for the engine room.